Another thing you can do is just drag and drop a ContextMenuStrip onto the form from the designer. Then click on the form and set its ContextMenuStrip property to the menu you dropped on it. When you drop a menustrip it will show up in the component tray at the bottom of the designer. Click on it and you can edit the menu at design time.

That way you won't need to handle the forms mouse up event and won't have to deal with adding the items programmatically.